The present invention is generally related to modulation systems. More specifically, the present invention is directed to a dual-mode open-loop modulator for a local oscillator capable of transmitting and receiving a communication signal.
The present invention includes modulation systems and techniques that balance four factors in radio design: cost, performance, current consumption, and size. Generally, improvement to any one of the parameters usually results in a detriment to at least one the remaining three. Obtaining the best design requires determining the best mix of these four factors.
One of the first decisions encountered in a new design is to decide which modulation technique to utilize. There are many approaches to choose from, ranging from ordinary amplitude modulation (AM) to more sophisticated and complex quadrature amplitude modulation (QAM). Based on research and experimentation with most of these techniques over the years, it is acknowledged that frequency shift keying (FSK) offers one of the best overall combinations related to cost, performance and low complexity.
FSK is easy to demodulate and utilizes a common limiter-discriminator circuit for demodulation. This demodulation technique has been around for 20 years and is easy to obtain, is inexpensive and works very well.
One major technical difficulty with utilizing FSK is encountered during the design of the modulator. Many wireless devices incorporate a frequency synthesizer to stabilize the frequency of the signal being transmitted by the unit. The synthesizer is xe2x80x9clockedxe2x80x9d to a stable crystal reference frequency. As a result, the output frequency is very stable over time and temperature. However, in an FSK modulated system, the object is to instantaneously vary the output frequency to convey information. Therefore, the FSK modulator must address two opposing design objectives. The FSK modulator must be able to vary its output frequency to convey information and, simultaneously maintain a very stable average center frequency over the course of the modulation period.
The following sequence of events commonly occur within the modulator circuit. See FIG. 1. Initially, a switch, e.g., a single pole single throw (SPST), is closed. This action causes the voltage control oscillator (VCO) output to lock to a frequency of Nxc3x97 (reference frequency). If a modulation voltage were to be fed into the loop filter at this point in time, the phase lock loop (PLL) would attempt to counteract the modulation voltage variations in an effort to maintain a constant output frequency. This inherently corrective characteristic of the PLL renders the circuit impractical as a modulator. However, if the switch is opened and the modulation voltage is then summed in at that moment, the VCO output frequency will vary as a function of the modulation signal without the corrective actions of the PLL being implemented.
The key factor to making this approach successful is timing. The moment the switch is opened, the voltage impressed across the loop filter (and VCO) begins to decay and drop to 0 volts. This sag translates into an undesirable frequency error (drift). However, if the open-loop period is kept short, i.e., less than a few milliseconds, then the VCO frequency drift can be kept to a minimum. Careful selection of the components for the loop filter in addition to a short open-loop period can produce frequency drift of less than 10 KHz in a few millisecond period.
To further reduce the cost of radios, direct modulation architecture can also be employed. This architecture utilizes a single local oscillator (LO) for both transmit and receive communication functions in the radio. See FIG. 2.
Note that the LO output is split to serve as both the transmitter signal source and as the first down-convert oscillator for the receiver. Radio design according to the present invention operates over a frequency range of 2.400 to 2.480 GHz and an intermediate frequency (IF) of 112.3 MHz. During a typical transmit period, the LO would be tuned to 2.44 GHz. Modulation would then be impressed onto the LO using the above mentioned open-loop technique. During the receive period, the LO can be tuned down by 112.3 MHz to provide an offset LO for the receiver. The incoming received signal (at the same frequency as the previously transmitted signal) is down-converted to an IF of 112.3 MHz and is then demodulated. The circuit used to tune and modulate the LO is shown in FIG. 3.
The transmission frequency chosen for implementation is based on many factors; such factors include: technology, capacity, reliability, government regulations, etc. In some instances, it may be preferable for a radio designer to utilize a frequency within the radio circuitry that is different than the frequency chosen for final transmission. In such cases, the radio designer may opt to work with an internal circuit frequency that is more manageable and easier to work with than perhaps an extremely high frequency selected for external transmission. In such cases, the internal circuit frequency can be adjusted prior to its external transmission by frequency doubling, halving or frequency heterodyning.
At the beginning of the transmit period, the synthesizer is in normal operating mode and the VCO is locked to a predetermined channel frequency. At a prescribed moment, the output of the synthesizer goes into a tri-state or high-impedance mode. This will effectively xe2x80x9copenxe2x80x9d the continuous loop and modulation is then applied to the VCO. After modulation has occurred, the transmit period is completed and the synthesizer re-tunes the VCO down by 112.3 MHz where it serves as the first LO in the receiver. The continuous loop is then xe2x80x9cclosedxe2x80x9d and will remain closed during the receive period.
A low-side LO injection may also be used wherein the local oscillator frequency is always tuned below the incoming frequency by 112.3 MHz. The VCO is designed so that an increase in tuning voltage results in an increase in output frequency. Therefore, the tuning voltage during the receive period will always be lower than the tuning voltage during the transmit period.
However, the use of multiple frequencies is susceptible to various problems. One such problem affects circuit capacitors in the loop filter and is known as xe2x80x9cdielectric absorption.xe2x80x9d Dielectric absorption produces an undesired xe2x80x9cmemoryxe2x80x9d effect in the capacitors of the loop filter. For example, a typical VCO may be tuned to approximately 1 V during the receive period. The receive period can extend to many milliseconds. During the receive period, the loop filter capacitor(s) are xe2x80x9csoakingxe2x80x9d at 1 V. During the transmit period, the VCO is re-tuned to a different voltage, i.e., 2 V. The re-tuning requires approximately 250 microseconds to complete. However, the moment the continuous phase lock loop is opened so that modulation can be applied, the voltage across the capacitor(s), i.e., C1 and C2, tends to fall to the previous xe2x80x9csoakedxe2x80x9d voltage level of 1 V. The loop is open at this point and no correction can be made to bring the tune voltage back to the appropriate value of 2 V. Moreover, correction is not desirable at this time because modulation is occurring. The voltage droop across the capacitor(s) is passed onto the VCO where it translates into frequency drift of the transmit signal. The drift can be severe, e.g., some capacitors have a high dielectric absorption and the resulting frequency drift falls outside the receiver""s passband within 100 microseconds.
There are capacitors available that exhibit very low levels of dielectric absorption. Utilizing these capacitors will reduce the voltage droop, and the resulting frequency drift, to manageable levels. However, these capacitors are in leaded form and are larger than desirable.
The present invention provides an effective way to reduce the effect of dielectric absorption in a dual-mode modulator utilizing different frequencies for transmitting and receiving communication signals.
Accordingly, an embodiment of the present invention is a method of providing an open-loop modulated carrier frequency to a local oscillator utilizing a receive mode and a transmit mode for signal communication. The method provides a phase lock loop comprising a synthesizer, a loop filter having a voltage across a capacitor, an op-amp circuit, a voltage control oscillator having an input voltage, and a modulator. The components of the phase lock loop are operably coupled together to form a continuous loop. The phase lock loop is capable of stabilizing a center frequency for cooperating with a transmit frequency, a receive frequency and an intermediate frequency. A tuning voltage applied to a voltage control oscillator determines the communication mode of the voltage control oscillator, i.e., transmit or receive. The transmission mode requires the phase lock loop to be opened. Modulation is impressed upon the center frequency for transmitting information. Independent of the communication mode, the voltage across the capacitor of the loop filter is maintained substantially constant. The substantially constant voltage across the capacitor reduces the adverse effect of dielectric absorption upon the capacitor, thus reducing frequency drift to the transmit frequency during open-loop modulation.
Another embodiment of the present invention is directed to a dual-mode open-loop modulator for a local oscillator utilizing a transmit mode and a receive mode communication frequency. The dual-mode open-loop modulator comprises a synthesizer for stabilizing a reference frequency. The synthesizer being operably responsive to the communication frequency mode. A loop filter having a voltage across a capacitor is operably connected to the output of the synthesizer. An op-amp circuit includes a first and second input and an output wherein the first input of the op-amp is operably responsive to the loop filter voltage. The second input of the op-amp is operably responsive to an offset voltage and the output of the op-amp circuit. A modulator circuit includes a voltage control oscillator having an input and an output. The input of the voltage control oscillator is operably responsive to a tuning voltage, the output of the op-amp circuit and a modulator. The output of the voltage control oscillator is operably connected to the synthesizer and the local oscillator. The loop filter voltage is maintained substantially constant during either communication frequency mode wherein the substantially constant loop filter voltage minimizes dielectric absorption of the loop filter capacitor, thus reducing frequency drift of the reference frequency during open-loop modulation.
A further aspect of the present invention includes an offset voltage applied to the second input of the op-amp circuit wherein the offset voltage is responsive to the communication mode of the modulator.
Another further aspect to the present invention is directed to the offset voltage applied during the transmit mode being substantially equal to the difference in tuning voltages respectively applied to the voltage control oscillator during the transmit mode and the receive mode.
Yet another further aspect of the present invention is directed to the offset voltage being correlated to a frequency offset substantially equal to a first intermediate frequency of the phase lock loop.
An object of the present invention is directed to alleviating the adverse effects of dielectric absorption in a dual-mode open-loop modulator utilizing a capacitor within its loop filter.
Other advantages and aspects of the present invention will become apparent upon reading the following description of the drawings and detailed description of the invention.